This invention relates to grounding system test apparatus and the like and particularly to apparatus which is used for adjusting the ground fault relay in multi-phase resistance grounded neutral power transmission systems and, with auxiliary apparatus, for locating faults in trailing cable power transmission systems.
In underground mining, such as sub-surface coal mines and the like, safety regulations require that the power supply in the mine be of the resistance grounded neutral type. Under such regulations the ground current is limited by the neutral grounding resistor for all voltages, for example, 480 volts, 600 volts, 995 volts, 4160 volts, 7200 volts and 13,200 volts, providing all connections and components are intact and in good working order. Major receptacles throughout the mine which are connected to the power system have a variety of devices that open the circuit breaker when ground current becomes excessive. Safety regulations provide that if the current exceeds one-half the limit value established by the neutral grounding resistor these devices must trip the breaker and disconnect the power source from the cable.
At the present time the generally accepted method for testing ground fault current is to insert a fused wire between phase and ground on the pins of an energized receptacle. Such a test is unsatisfactory because it does not insure that the grounding resistor is limiting the current or that the current level is below the acceptable limit. This test also exposes personnel to hazardous voltages which are present on the pins of the receptacles after jumper wires and/or filters have been inserted to enable the ground conductor continuity monitoring circuit, which also controls the tripping of the circuit breaker. Trailing cable power systems have a ground conductor continuity monitor of some sort which is interlocked with the circuit breaker tripping apparatus. The ground conductor continuity monitoring circuit emcompasses the ground conductor, a return path for current therethrough, means for transmitting a current through ground conductor and return path and means for detecting that current. The return path may be a pilot conductor in the trailing cable and the current may be of power frequency. Such systems are called impedance systems. If a monitoring current of frequency higher than the power frequency is employed the return path may be through the phase conductors and that current is taken off through a three-phase inductivecapacitive filter or tuned circuit. Such systems are called audible systems because the monitoring current frequency is usually in audible range.
The interlock prevents the circuit breaker from closing unless the ground conductor continuity monitor indicates continuity. In order to test the ground fault protective apparatus it is necessary to actuate the ground conductor continuity monitor so that it does indicate continuity.
When cable faults occur it is necessary to determine the location of the fault in the cable before it can be repaired If a damage area is not discovered by quick visual inspection the first step to finding a phase-to-ground short is to cut two existing splices and then check continuity with a meter. This method is time consuming and costly due to production loss and the cost of repair parts. The two other methods sometimes used, which are in violation of state and federal regulations, are:
1. Using DC current from resistance type bond welders or trolley circuits connected from phase to ground.
2. Closing the circuit breaker with the faulty phase and ground conductors connected phase-to-phase.
In recent years cable fault detector systems have become commercially available which avoid the dangers above mentioned. Those systems comprise a transmitter which supplies a relatively low voltage signal and a detector therefore. The transmitter is connected to the faulted phase and ground conductors and the transmitter signal may be of power line frequency or higher frequency. The detector is a field strength monitor which detects and indicates the strength of the electromagnetic field surrounding the cable due to the transmitter signal current. The detector need not be conductively connected to the cable but is merely held against it so as to be inductively coupled therewith. It can therefore be moved along the cable and continues to receive signals as long as the signal current is present. The voltage output of most of those transmitters will not pass enough current through high resistance faults to generate a detectable field.